1. Field of the Invention
The present invention relates to a fuel injection device in which fuel may be stepwise injected.
2. Description of Related Art
Conventionally, in a fuel supply system in which fuel is supplied from a high pressure supply pump to an injector that is a fuel injection device, a technology that a needle lift is varied by a value of fuel pressure to change its injection characteristic has been proposed. Injection rate, atomization density and distribution behavior of fuel affect largely on fuel ignitability, formation of NOx, black smoke, HC and the like and combustion efficiency.
For example, well known is a nozzle with two-stage valve opening pressure that has two springs for biasing a needle with a predetermined needle lift interval. According to this technology, the needle lifts due to pressure of fuel delivered by a fuel injection pump. However, a value of pressure of fuel delivered to the fuel injection device from the fuel injection pump becomes variable according to engine operations. Therefore, it is difficult to always realize an optimum injection rate demanded by the engine over an entire range of engine operations.
To cope with this problem, an injector 230, as disclosed in U.S. Pat. No. 5,694,903 and shown in FIG. 42, is known. The injector 230 is provided with a control chamber 260 by which fuel pressure is applied to a needle 231 in a direction of closing an injection hole. A lift of the needle 231 is controlled by making a force acting in a direction of opening the injection hole due to fuel pressure transmitted to a fuel accumulating space 232 larger or smaller than a sum of forces receiving in a direction of closing the injection hole due to the fuel pressure of the control chamber 260 and biasing force of a spring 237. Even if the fuel pressure is varied according to the engine operations, regulating pressure of the control chamber 260 accurately controls an opening and closing timing by the needle 231.
Further, a lift of a pilot valve stem 270 is controlled with two steps by biasing forces of two springs 290 for urging the pilot valve stem 270 in a direction of closing the control chamber 260 and an attracting force of a coil 274. As a result, it is intended that the needle 231 is stepwise lifted to secure a predetermined fuel injection rate.
However, the conventional fuel injection device has a drawback that, even if the stem 270 is stepwise lifted, the needle is not always stepwise lifted simultaneously with the stem 270, since the needle 231 is lifted when a value of the fuel pressure of the fuel accumulating space 232 exceeds a sum value of pressure of the control chamber 260 and biasing force of the spring 237. Further, if the electromagnetic attracting force of the coil 274 is varied due to, for example, a change of temperature, a lifting characteristic of the stem 270 such as an opening area characteristic of the stem 270 is forced to change. Furthermore, due to a characteristic change of fuel such as viscosity, the pressure of the control chamber 260 is changed unstably. Accordingly, a lifting characteristic of the needle 231 is also changed so that the fuel injection rate may become unstable. Moreover, since a lifting control amount of the stem 270 is very small, it is difficult to secure a uniform quality in each of the injectors 230 so that an accurate and stable injection control may not be realized.
In the conventional fuel injection devices, though the injection rate may be variably controlled so far, it is impossible to realize a variable control of fuel atomization event such as atomization angle and droplets reaching distance.
Inadequate control of the atomization event causes to harm fuel consumption and an output so that NOx, black smoke, HC and the like may be more formed.
Further, as shown in JP-A-10-54323, well known is a fuel injection valve in which control valves are arranged at an inlet portion through which high pressure is introduced to the control chamber and at an outlet portion through which high pressure is released from the control chamber, respectively. With the plurality of control valves, the lift of the needle is stepwise controlled to obtain the stable lift control, while the leak amount can be reduced, since respective opening and closing controls of the inlet and outlet of the control chamber can be independently controlled.
However, the injection valve mentioned above still has a drawback that the valve becomes larger and is expensive since pluralities of electromagnetic valves are necessary.